Ultrastructural morphometric of gap junctions during differentiation of stratified squamous epithelium.

The presence of gap junctions in stratified epithelia has now been extensively documented, but there have been few attempts to quantify them. In the present report, samples of hamster cheek pouch mucosa were processed for electron microscopy and electron micrographs from defined basal, spinous and granular layers were obtained. Using a combination of direct measurement and stereological intersection counting techniques, the relative surface areas of peripheral gap junctions (i.e. those in direct contact with the epithelial plasma membrane) and annular gap junctions (i.e. those present as complete, approximately circular profiles within the epithelial cell cytoplasm) were determined. Following estimation of the plasma membrane surface area of 'average' epithelial cells from each of the defined strata, relative values were transformed into absolute data. Data from peripheral and annular junctions were pooled to provide an estimate of total gap junctions area. Relative surface area estimates were similar for peripheral, annular and total gap junctions, in that values were invariably highest in the spinous layer and lowest in the granular layer. Absolute data indicate that there is more than a threefold increase in the area of membrane differentiated into gap junctions in the average spinous cell when compared with the average basal cell. Values for total gap-junctional areas in the average granular cell are reduced somewhat with respect to the average spinous cell and this is effected by a decrease in the area of peripheral gap junctions. We conclude that there is synthesis of gap junctions between basal and spinous cells, which is followed by evidence of degradation between spinous and granular cells. The magnitude of the estimates of area is comparable to those obtained from other stratified and non-stratified epithelia and it would thus appear that gap junctions may play a significant role in cellular control processes in all viable epithelial strata.